Imaging apparatuses utilizing the effect of nuclear magnetic resonance (NMR), such as NMR tomographs, include a magnetic system for generating a static magnetic field (B.sub.0 field) which serves to align the nuclear spin and generally has a predetermined gradient structure, and a coil arrangement for generating a spatially homogeneous high-frequency field (B.sub.1 field) which extends generally perpendicular to the static magnetic field and which permits tilting of the spin vectors aligned by the B.sub.0 field by a predetermined angle or flipping of the spins and serves to detect a high-frequency generated thereby and radiated by the sample.
The aforementioned coil arrangement serving to transmit and receive may include a single coil. However, to obtain a high local resolution with the aid of NMR apparatuses for three-dimensional imaging, as is known it is advantageous to employ several coils, that is a transmitting coil and at least one receiving coil. The transmitting coil, the so-called "homogeneous coil" is generally energized by an HF pulse and generates in the sample the homogeneous magnetic field B.sub.1 for stimulating the nuclear magnetic resonance. With the receiving coil an NMR-signal following the stimulating pulse and induced in the sample is received which is amplified in a preamplifier and passed on to a following signal processing circuit. The receiving coil has smaller dimensions than the transmitting coil and may be constructed as so-called "surface coil" which is arranged as near as possible to the surface of an object to be investigated ("sample") over the region to be measured.
The application of this method is restricted by the presence of the surface coil in the field region of the transmitting coil distorting the HF field of the homogeneous coil particularly in the region to be measured. Conversely, the NMR signals are disturbed by the transmitting coil, although to a lesser extent.
It is known from the publications M. A. Packard, Rev. Sci. Instr. 19, 435 (1948) and A. Haase et al., J. Magn. Res. 56, 401 (1984) to decouple the transmitting and receiving coils from each other by a perpendicular arrangement. However, this step cannot always be employed; thus, it is necessary for example in medical diagnostic uses to be able to arrange the surface coil at any desired angles to the transmitting coil.
It is further known from the publication by C.-N. Chen et al., in J. Magn Res. 54, 324-327 (1983), to inductively decouple the transmitting and receiving coils from each other by metal strips (paddles) arranged therebetween. The metal strips must be disposed in the interior of the probe head and are therefore difficulty accessible. Since however every change of the coil arrangement requires resetting of the metal strips this method is not suitable for practical uses.
In the publications by R. J. Blume in Rev. Sci. Instr. 33, 1472 (1962) and by R. R. Lembo et al., in J. Phys. E. (Sci. Instr.) 8, 632 (1975) circuits are described which compensate the voltages which are induced in the coils due to their coupling and cause the aforementioned field distortions. In this case as well, when the coil arrangement is modified a balancing of the compensation circuits is necessary. In addition, such circuits are complicated and troublesome.
Finally, it is known to prevent the reaction of a coil passive at that time (i.e. not transmitting or receiving) by detuning its resonance circuit and thus reducing its coupling to the HF field of the other coil. In the publication by M. R. Bendall in Chem. Phys. Lett. 99, 310 (1983) it is known in this connection to connect an oppositely parallel-connected diode pair either in series or in parallel to the resonance circuit of the coil to be decoupled in order to detune the coil resonance circuit either at high or at low signal voltage. The coil resonance circuits are however generally made very broadband so that the effectiveness of this step is small.
It is known from the publications by D. I. Hoult in Progr. NMR Spec. 12, 41 (1978) and J. L. Lowe et al., in J. Sci. Instr. (J. Phys. E) 1, 320 (1968) for arrangements having only a single coil serving both as receiving and as transmitting coil to protect the input of a receiving part connected to said coil from the HF transmission pulse by a pair of diodes connected in parallel with opposite poling. The diode pair is connected via a .lambda./4 line to a T branch of a connection line between an HF transmitter and the coil, .lambda. designating the wavelength corresponding to the operating frequency of the coil. The diode pair short-circuits for the duration of the HF transmission pulse the input of the receiving part and the one end of the .lambda./4 line so that at the resonance frequency and in the steady state the branch to the receiving part does not take any current. However, no suggestion can be derived from these publications as to how a coupling between two coils and a reaction of a coil on an external field can be suppressed.